With present day jet aircraft, structures typically known in the industry as “chevrons” have been researched to attenuate noise generated by a jet engine. The chevrons have traditionally been fixed (i.e., immovable), triangular, tab-like elements disposed along a trailing edge of a primary and/or a secondary exhaust nozzle of the jet engine nacelle such that they project into the exhaust gas flow stream exiting from the exhaust nozzle. The chevrons have proven to be effective in reducing the broadband noise generated by the mixing of primary-secondary and secondary/ambient exhaust streams for high thrust operating conditions. Since the chevrons interact directly with the exhaust flow, however, they also generate drag and loss of thrust. Consequently, there is a tradeoff between the need to attenuate noise while still minimizing the loss of thrust due to the presence of the chevrons,
Noise reduction is typically needed for takeoff of an aircraft but not during cruise. Thus, any noise reduction system/device that reduces noise at takeoff (i.e., a high thrust condition) ideally should not significantly degrade the fuel burn during cruise. A compromise therefore exists between the design of static (i.e. immovable) chevrons for noise abatement and the need for fuel efficient operation during cruise.
Thus, there exists a need for a noise reduction system which provides the needed noise attenuation at takeoff but does not produce drag and a loss of thrust during cruise conditions. More specifically, there is a need for a noise reduction system which permits a plurality of chevrons to be used in connection with an exhaust nozzle of a jet engine to attenuate noise during takeoff, but which also permits the chevrons to be moved out of the exhaust gas flow path of the engine during cruise conditions to prevent drag and a consequent loss of thrust during cruise conditions.